Corrosion | Mandatory Practical Activity 19 | Prevention of Rusting
Corrosion is the formation of undesirable metallic compounds at the surface of pure metals or alloys.
The term corrosion was once limited to the destruction, loss or wearing away of a metal due to chemical action.
Very few metals are found pure in nature.
Most metals are very reactive and will react with other substances they come in contact with in their environment especially the atmosphere, water and moist solids. Oxygen, water, carbon dioxide, ions in solution and hydrogen sulpide are the main substances involved in corrosion.
Examples of Corrosion
Rusting is the corrosion of iron. The iron is converted to hydrated iron oxide due to its reaction with oxygen and water. This ‘rust’ is a loose brown-orange insoluble solid.
Tarnishing of silver due to the formation of black silver sulphide due to the reaction with hydrogen sulphide.
The formation of the green deposit called verdigris (copper carbonate) on copper and bronze.
Rusting is a Chemical Process
A chemical process is one in which a new substance is made that has new chemical properties. During rusting iron is changed to a hydrated iron oxide.
Iron + Water + Oxygen » Hydrated Iron Oxide (Rust: Fe2O3.3 H2 O)
The rust layer does not protect the metal.
Rust is loose material which easily flakes away exposing fresh metal which then rusts.
The rust is also permeable to water and air and so rusting continues under the rust layer finally leading to its complete destruction.
Conditions Necessary for Rusting
Water and oxygen must both be present for rusting to happen.
Mandatory Practical Activity 19
Carry Out an Experiment to Demonstrate that Oxygen and Water are Necessary for Rusting.
1. Place three iron nails in a test tube with some white dry calcium chloride solid.
The top of the test tube plugged with some cotton wool.
Calcium chloride absorbs water vapour from the air and so the air is dry – water is not present.
2. Place three iron nails in a stoppered test tube of boiled water with a layer of oil on top of the water.
The water was boiled for 15 minutes to drive off all the dissolved oxygen.
The oil prevents oxygen from the air dissolving in the water.
3. Three nails are placed in an open test tube containing some water.
4. Allow the tubes to stand in a beaker or test tube rack for a few day and examine for rusting.
5. Results: rusting only occurs in tube 3; no rusting without water or without oxygen.
6. Conclusion: water and oxygen are together needed for rusting.
Prevention of Rusting: Painting, Oil and Galvanising
Painting the metal places on its surface an impermeable barrier to water and air.
Therefore the metal cannot come in contact with water and oxygen and so rusting cannot take place.
A layer of oil is placed on the surface of the metal.
Water and oil do not mix and so water is repelled from the surface.
Without water rusting cannot occur.
Galvanising is the coating of an object with zinc.
The zinc coating may be done by electroplating or dipping it into molten zinc.
The zinc becomes covered with a thin layer of zinc oxide on its reaction with oxygen of the air.
The zinc oxide forms an impenetrable protective layer preventing further corrosion.
Even if scratched the object will not rust until all the zinc has been removed.